The present invention relates generally to the field of corrosion protection for metal substrates, and more specifically to bonding solutions and coating compositions free, or substantially free, of carcinogenic or toxic metals.
Compositions comprising phosphoric acid and aluminum metal are well known for use in protecting metallic surfaces such as ferrous surfaces from corrosion. In such coating compositions, particulate metallic aluminum, in the form of flake and/or powder, for example, is combined with a phosphoric acid bonding solution to form a coating composition which is then applied to the metallic surface being treated. After application of the coating to the surface, it may be heated to a first temperature, generally upwards of 500xc2x0 F. (260xc2x0 C.), until the coating is rendered essentially water insoluble. Then the coated surface may be cured at a second temperature, generally above 1000xc2x0 F. (538xc2x0 C.) to form the final protective coating.
It is often further desirable to provide an extra protective barrier to the metal surface that may provide thermal resistance or simply augment the corrosion protection afforded by the coating and bonding solution described above. In such case, the coating resulting from the combination of particulate metallic aluminum and phosphoric acid bonding solution is termed an xe2x80x9cundercoatxe2x80x9d or xe2x80x9cbasecoatxe2x80x9d. An extra protective layer applied to the cured undercoat is termed a xe2x80x9ctopcoatxe2x80x9d. The topcoat may be formed from a bonding solution similar to that used in the undercoat, but containing little or no particulate metal. The result, upon application and curing, is a glassy, ceramic-like layer that provides water resistance, thermal resistance, and augmented corrosion protection. Such a topcoat composition, as known in the art, often contains chromate. The topcoat bonding composition may further contain a pigment which imparts visually aesthetic qualities to the coating. The pigment(s) may also be functional and improve certain properties such as corrosion resistance, erosion life, and bond strength.
Though basecoat coating compositions contain particulate aluminum metal, care must be taken in the preparation of phosphate-based coatings. The phosphoric acid bonding solution can react with the aluminum. Such reactions are considerably exothermic and can be very violent, causing the aluminum powder to burn or even explode. These reactions may result in the conversion of the metallic aluminum into various salts. Protective topcoats, though not containing particulate aluminum metal, are equally susceptible to reaction with metallic aluminum because protective topcoats are directly applied to metallic aluminum-containing basecoats. In either case, such reactions interfere with the formation of suitable protective coatings. Thus, the reactive stability of a coating formulation in the presence of metallic aluminum is of foremost concern.
U.S. Pat. No. 3,248,251 to Allen, describes coating compositions consisting essentially of a slurry of solid inorganic particulate material (such as metallic aluminum) in an aqueous acidic bonding solution containing dissolved metal chromate, dichromate or molybdate, and phosphate. Allen discloses that the addition of chromates or molybdates to the acidic bonding solution effectively passivates the solution toward aluminum and inhibits the oxidation of metallic aluminum, allowing particulate aluminum to be combined with the bonding solution without the undesirable chemical reaction between the acidic bonding solution and the aluminum.
These xe2x80x9cAllenxe2x80x9d coatings have been, and still are, successfully used to provide high quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures. It is also known that the inclusion of chromium or molybdenum in the coating composition, whether used in corrosion resistant basecoats or protective topcoats, provides a coating having improved corrosion resistance.
However, while chromates and molybdates have been used successfully to reduce the reactivity of the aluminum in such coating compositions and to improve the corrosion resistance in the coatings, the use of chromates and molybdates has become a problem because of environmental considerations. Chromates are considered carcinogenic. Molybdenum is classified as a toxic heavy metal. It is therefore desirable to avoid the use of solutions of their salts, or at least to reduce their use. For this reason, it has become desirable to develop a phosphate/aluminum corrosion resistant basecoat composition which requires little or no chromate or molybdate to control the reactivity between the acidic phosphate bonding solution and the particulate aluminum added thereto. Similarly, it has become equally desirable to develop a protective topcoat having little or no chromate or molybdate. Such coating compositions should protect ferrous metal alloy surfaces from oxidation and corrosive environmental conditions, especially at high temperatures, approximately as well as and preferably better than the so-called Allen coatings.
Efforts have been made to exclude chromate and molybdate from coating compositions while maintaining stable formulations. U.S. Pat. No. 5,242,488 to Stetson et al., describes a basecoat coating composition for ferrous alloys which does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered aluminum. The composition consists essentially of a slurry mixture of a bonding solution and aluminum powder. The bonding solution consists essentially of water, phosphoric acid (H3PO4), and aluminum ions. The bonding solution must contain sufficient aluminum ions in solution so that it is substantially equilibrated with respect to aluminum metal pigments, i.e., the amount of aluminum in solution must be substantially at the saturation point, thus leaving the bonding solution essentially inert with respect to any subsequent additions of aluminum.
This Stetson patent also teaches that magnesium may desirably be used to at least partially neutralize the aqueous phosphoric acid mixture, either before or after equilibration of the mixture with aluminum. The magnesium compound used is either MgO or MgCO3. All examples given in the patent utilize magnesium ions.
U.S. Pat. No. 5,279,649, also to Stetson, et al., discloses substantially the same compositions to which V2O5 has been added to produce vanadate ion, adding another inhibitor to the aluminum equilibrated mixture. Addition of V2O5 is an example of the addition of a toxic substance, listed on the OSHA extremely hazardous substance list and also subject to Clean Air Act and CERCLA regulation.
These Stetson coating compositions are designed to avoid the use of chromate and molybdate ions and require the bonding solution to be equilibrated with respect to further additions of aluminum as described in these patents.
Although the Stetson patents indicate that these formulations control the reactivity between the bonding solution and the aluminum, some reaction still occurs between the bonding solution and the powdered aluminum when the slurry compositions of the Stetson patents are formulated.
U.S. Pat No. 5,478,413 to Mosser et al. is directed to coating compositions lacking chromium or molybdenum. These coatings are pigmented with aluminum powder and can be applied to all ferrous alloys. These coatings require that magnesium ion be present in a bonding solution to promote stability toward the aluminum metal, and that the pH of the composition be between 2.0 and 4.5.
Chromate-free coating compositions of the prior art suffer the disadvantage that composition stability decreases, often dramatically, as pH is lowered below, for example, 2.0. This is unfortunate since lower pH values generally promote better dissolution of constituent species and better film-forming coating compositions. In attempting to raise pH to more acceptable, higher levels (e.g., greater than 2.0), the risk is conversely run that certain ionic species may precipitate out of solution.
It is therefore desired to formulate a chromate- and molybdate-free bonding composition, or one which is of reduced chromium and molybdenum content, which not only has a reduced reactivity with particulate aluminum when the two are combined to form a coating composition, but that is also stable at lower pH values. Such a bonding solution should also form, upon curing, a hard, glossy surface. Such a bonding solution should also bond to, but not react with, ferrous alloys to the extent that a conversion coating or a Fe-phosphate layer (such as described in von Fraunhofer, J. A. Basic Metal Finishing, 1976 (Chemical Publishing Co.: New York), pp. 146-151; incorporated herein by reference) would be produced. The present invention provides such a composition.
A heat curable bonding composition for providing a coating to a solid substrate is provided, comprising water, phosphate ions, borate ions, and aluminum ions. The bonding composition has a pH in the range from about 1.4 to about 2.2 and the composition is substantially free of chromate ions, molybdate ions, and magnesium ions.
A heat curable coating composition is provided comprising metallic aluminum particles dispersed in a bonding composition, the bonding composition comprising water, phosphate ions, borate ions, and aluminum ions, wherein the amount of aluminum ions in solution is less than necessary to equilibrate the solution with respect to the metallic aluminum particles. The composition has a pH in the range from about 1.4 to about 2.2 and is substantially free of chromate ions, molybdate ions, and magnesium ions.
In an alternative embodiment of the invention, a heat curable coating composition is provided, comprising metallic aluminum particles dispersed in a bonding composition. The bonding composition in such embodiment comprises water, phosphate ions, borate ions, magnesium ions, and aluminum ions. The amount of aluminum ions in solution is less than necessary to equilibrate the solution with respect to the metallic aluminum particles. The bonding composition has a pH in the range greater than or equal to 1.4 and less than 2.0, and the coating composition is substantially free of chromate ions and molybdate ions. In such an embodiment of the invention, the bonding composition is stable with respect to the metallic aluminum particles for a least one hour under ambient conditions.
The bonding compositions of the invention may be employed as heat curable topcoat coating compositions. The bonding compositions of the invention may be combined with metallic aluminum particles in heat curable compositions, especially for use in basecoat coating applications.
An article of manufacture is provided comprising a substrate with a coating, the coating prepared by a method comprising the steps of providing a heat curable coating composition of the invention, applying the coating composition to the substrate, and heating the substrate thereon to form a heat cured coating. The coating composition may be a topcoat coating composition or a basecoat coating composition.